Embodiments of the present invention generally relate to a grease delivery system, and more particularly, to a well intervention module, an underwater hydrocarbon extraction facility comprising such a grease delivery system, and a method of delivering grease to a location.
It is well known that when performing wire-line intervention operations at underwater facilities, for example subsea hydrocarbon extraction facilities incorporating at least one hydrocarbon production well, it is necessary to inject high pressure grease into an intervention pressure control head during the wire-line operations in order to lubricate the line and provide a seal against ingress of well fluids. Previous grease injection attempts made use of a grease supply from the surface (i.e. the top-side), which was pumped subsea to the facility. However it is understood that this approach encountered grease control and response issues. For example, the viscosity of the grease is highly temperature dependent, and since the grease is pumped a long distance subsea, the ambient temperature will vary significantly over the distance, which complicates control.
To avoid such grease control and response issues, it is optimal to control the grease supply locally to the injection point. In other words, a supply of grease is stored subsea, thus avoiding temperature differential problems, and applied directly where needed. A problem arises with such a system however such that only relatively small quantities can be stored subsea, typically in grease tanks. Since the grease supply must remain as constant as possible, it is advantageous to provide a system whereby switchover from one tank to another (for example, a replacement tank) may be as seamlessly performed as possible, with any grease flow interruption minimized. US 20110094731 A1 describes such a local grease injection system, which performs switchover using near-instantaneous switching of hydraulic control valve(s) located in the grease supply line between the tank(s) and the injection point.
While that system itself provides advantages over the previous top-side grease supply methods, it does suffer from a number of problems. For example, the system is reliant on reliable and instantaneous switching of the hydraulic valves; if this fails then the system may be severely compromised. Furthermore, while the switching is near instantaneous, small gaps in the flow may still occur even during fault-free operation.